Recently, in silver plating electronic parts, high-speed plating process has been in wide use for economic reason. The process uses a high-temperature plating solution with a high silver concentration at a high flow rate to obtain a high silver plating rate. In the high-speed silver plating solution for the process, the free cyanide concentration must be kept properly low. This is because, at a high concentration, the free cyanide will vigorously decompose at an elevated temperature, evolving a dangerous volume of hydrogen cyanide gas. Thus, the high-speed silver plating solution is characterized by the introduction thereinto of a silver salt in the form of an alkali silver cyanide and the provision of a pH buffer system which maintains the pH of the solution in the range of about 7.5 to about 9.0 so that the free cyanide produced from the cathode during the electrolysis is gradually decomposed and removed to prevent its buildup in the solution. The term "free cyanide" as used herein means a cyanide which does not form a complex with a metal ion.
More recently, high-speed partial plating, whereby only the substrate portions desired to be plated are selectively plated at a high speed, is finding acceptance for more effective utilization of the expensive silver. This high-speed partial plating is essentially based on a jet plating technique which comprises masking the substrate portions that need not be plated, pumping a silver plating solution at a high speed and directing it against the exposed portions to be plated, while allowing a current to flow across the substrate and an insoluble anode, thereby to effect silver plating.
A major problem of the high-speed silver plating, particularly the high-speed partial silver plating has been the occurrence of deposition of silver on the less-noble metal substrate by displacement reaction by mere immersion of the substrate in the silver plating solution because of high silver concentration in the plating solution. This deposition of silver by displacement reaction occurs markedly with such metal substrate as copper, copper alloys, copper-plated base metal, iron, ferroalloys, nickel and nickel alloys. The deposits of silver by displacement usually exhibit poor adhesion to the substrate and cause subsequently formed electrodeposits of silver to scale off or, upon heating, blister or tarnish, resulting in defective plating. This is fatal, especially, for the silver plated electronic parts. Moreover, in partial plating, even the portions that need not be plated become plated, consuming the expensive silver wastefully. A further disadvantage is the contamination of the plating bath with ions of copper or other less-noble metal that has dissolved out of the substrate by the displacement reaction with silver.
In order to prevent the formation of low-adhesion silver deposits by immersion, it has been customary to form a thin silver plated layer known as strike from a plating solution with a low silver concentration and then deposit an ordinary plated layer thereon. However, such two-stage plating operation is cumbersome. In the case of partial plating, the strike improves the final plated deposit adhesion to the substrate but necessarily forms the strike layer on the substrate portions that need not be plated, with a loss of silver.